1. Field of the Invention
The present invention relates to a tracking control device adapted for use in a video tape recorder (VTR) to correct any tracking error by changing the positions of magnetic heads by means of bimorphs.
2. Description of the Prior Art
FIG. 21 illustrates a track pattern in the D-1 digital VTR format (525/60 system).
In the diagram, there are shown a magnetic tape 1, a cue track 2, a control track 3 and a time code track 4. A servo reference signal is recorded on the control track 3 at an interval of four video tracks.
Denoted by 5U and 5L respectively are a video upper sector and a video lower sector which constitute video tracks. Here an aggregate pattern of tracks is termed "sector". Shown in FIG. 21 are tracks of one field.
In the 525/60 system, a 1-field video signal is composed of 20 sectors, where each field begins at an upper sector 5U and terminates at a lower sector 5L. Each field is divided into 5 segments, and data of 1 segment is stored in 4 sectors shaded in the diagram. More specifically, data of 1 segment is recorded in the 4 sectors astride the 4 tracks A to D.
In such arrangement, merely a quarter of one segment is harmfully affected if one of the 4 tracks is rendered unusable due to some damage to the head or if the error rate is extremely deteriorated. Furthermore, since the video signal is sequentially distributed to the 4 tracks (sectors) in conformity with the time series, the error correcting function can be fully performed even when the data of any one track is lost.
Denoted by 6 is an audio sector disposed substantially at the center of the tape 1 to form audio tracks. Selection of such position is based on the reason that, even if any skew occurs in the tape, the center is affected minimally.
FIG. 22 illustrates an arrangement of audio sectors, wherein 4-channel audio data temporally concomitant with the video data of 2 shaded segments are recorded in 16 sectors. The length of the error correction block is reduced to minimize the undesired possibility that the data in any sector unconcerned with editing is read and rewritten. Such reduction of the error correction block length diminishes the capability of correction for flaw in the longitudinal direction of the tape or dropouts along the tracks, but such demerit is compensated by writing the same data twice.
FIG. 23 shows an exemplary rotary magnetic head device, wherein a rotary drum 7 is equipped with channel-A playback magnetic heads Ha, Hb, Hc', Hd' and channel-B playback magnetic heads Hc, Hd, Ha', Hb' disposed substantially at angular intervals of 180.degree.. The distance d between the heads is set in conformity with the track pitch in such a manner that, when the head Ha scans the track A, the heads Hb, Hc', Hd' scan the tracks B, C, D respectively (FIG. 24). The same setting is applied to the heads Hc, Hd, Ha', Hb' as well.
In a playback mode, the heads Ha, Hb, Hc', Hd' (Hc, Hd, Ha', Hb') scan the four tracks A, B, C, D respectively so that both the video data and the audio data are reproduced therefrom. In FIG. 23, recording heads are not shown.
In a normal 1-fold speed playback mode, the scanning angle of each head on the tape 1 becomes equal to the angle of the track recorded on the tape. However, in a playback mode at any different speed, the scanning angle of the head fails to be coincident with the track angle, so that there occurs a track deviation (inclination error) which appears to be guard band noise on the reproduced image. FIG. 25 illustrates scanning loci of the heads in a -1-fold speed mode, a still playback mode, a +1-fold speed mode and a +2.5-fold speed mode.
For the purpose of eliminating such track deviation, there is proposed a method of firmly attaching each head to a bimorph and controlling the head position by means of the bimorph. For driving the bimorph in this case, there are required a track inclination waveform signal, a tracking waveform signal, a ringing waveform signal and a tracking error correction waveform signal.
In any mode other than a +1-fold speed mode, each head scans the recorded track on the tape obliquely at a predetermined angle of inclination conforming with the tape speed. Therefore it is necessary to change the head position in a manner to correct the angle of inclination. The track inclination waveform signal is used for changing such position.
At any speed other than a +1-fold speed, it is fundamentally impossible to maintain the fixed relationship between the head and the track. Therefore a correction needs to be executed by changing the head position in accordance with the tape transport position. The tracking waveform signal is used for execution of such correction.
When the bimorph is driven, there may occur unrequired vibration by hysteresis or mechanical resonance to consequently disorder the tracking. The ringing waveform signal is added during a blanking period (non-scanning time) so as to correct such unrequired vibration.
And the tracking error correction waveform signal is used for optimizing the other tracking.
In the execution of tracking under predictive control, there exists the possibility that the tracking is disturbed by a variety of factors including deterioration of the circuits, temperature characteristics, and conditions of the playback tape.
Correction of any track deviation with a shift of the entirety as in a +1-fold speed mode can be achieved by correcting the DC component.
However, in any different-speed playback mode, some track deviation is caused also by other factors such as the predicted value of the track pitch gain for obtaining a tracking waveform signal, and the predicted value of the track inclination gain for obtaining a track inclination waveform signal.
Correction of the track deviation resulting from such predicted values is not achievable by mere addition of the DC component, and such means fails to attain complete correction of the track deviation.